Ice an inch thick is expected to descend on parts of the US from Texas north to the Ohio Valley, as an ice storm prompts flight cancellations at Dallas-Fort Worth airport on Friday and the closings of schools and many public offices.

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Chicago — An ice storm creeping north from the southern Great Plains and toward the Ohio Valley is already causing flight cancellations, traffic accidents, school closings, and overall treacherous conditions that are expected to last through the weekend.

The storm, which The Weather Channel is naming Cleon, originated late Thursday in Texas and stems from a perfect syncing of conditions: A low-lying cold air mass moving south and east will meet a higher level of warm air moving off the Gulf of Mexico.

“So precipitation falls as rain, and when it gets down to the cold layer, it turns into sleet, and once it hits the ground, it turns into ice,” says Ken Clark, expert senior meteorologist for AccuWeather, who is based in Rancho Cucamonga, Calif. “We’re looking at travel woes for the next couple of days.”

Ice storms are relatively common winter events, considered more treacherous than snow because they tend to make travel more dangerous and cause power outages that can last up to a week. Thursday’s storm, which is slated to move into central Oklahoma, southwest Missouri, and then the southern Ohio Valley, is “stronger than the typical winter storm,” Mr. Clark says. The heaviest snow will fall across the Ohio Valley into the mid-Mississippi Valley.

Temperatures in the central and western US will be 10 to 30 degrees F. below normal for several days. In some areas of the northern Plains, high temperatures will remain below zero.

“This is brutally cold air,” Clark says.

For this storm, an inch or more of ice is expected to coat trees, power lines, and roads. Ice at least a quarter-inch thick that collects on surfaces constitutes an "ice event," according to the National Weather Service.

A second storm is expected over the weekend, originating over the Rocky Mountains and moving across the Ohio Valley and into the Mid-Atlantic and the Northeast by late Saturday or early Sunday.

Ice storms are most prevalent in the Northeast, according to research from the University of Albany, the State University of New York, from March 2012. These types of storms do not trend in any particular way: Since 1993, the iciest winter came in 2002-03, when there were 15 ice storms. The previous winter chalked up the fewest: three. The official ice storm season stretches from December through March, with ice storms most likely to occur in January.

In Texas's Dallas-Forth Worth area, bridges and overpasses had iced over as of early Friday morning, as temperatures hovered around freezing. About a quarter-inch of ice will be on the ground by sunrise, says the National Weather Service. Temperatures in the area will remain at or below freezing through late Saturday.

Areas of northwest Dallas already experienced power outages as of late Thursday. The Dallas Independent School District canceled Friday classes and Saturday events. Other institutions such as the University of North Texas in Denton, the Dallas Zoo, and Dallas County courts and offices also are closed Friday and Saturday.

Texas Gov. Rick Perry (R) issued an emergency proclamation late Thursday that dispatched 54 Texas Military Forces personnel and 20 supporting vehicles in Wichita Falls and Sherman to help and rescue stranded travelers.

By 7 a.m. Friday morning, power outages in Texas affected at least 236,000 customers, from the northwest part of the state south to Austin, according to energy company Oncor, which supplies power to 3 million homes in Texas. Entergy Arkansas, which supplies power to more than 700,000 homes in Arkansas, said about 6,000 homes are without power.

Both companies are receiving help from energy companies in neighboring states, including Florida Power & Light Co. in Miami, which announced Thursday it is sending more than 500 workers to both states to help restore power.

By early Friday, at least 1,293 flights from around the US scheduled to travel in and out of Dallas-Fort Worth Airport were canceled. The majority of those were American Airlines flights; the airline says all flights connected with the airport were canceled through 11 a.m. central time Friday.

The Dallas Morning News reports accidents on surrounding interstates and thoroughfares, including an 18-wheel truck that got stuck on the I-20 westbound ramp to US 67. By Thursday night, multiple accidents were reported on a Highway 66 bridge near Fort Worth, and a pile-up occurred near the Sam Rayburn Tollway. Icing also caused vehicle accidents in Missouri and southern Illinois.